In the humidification process, steam is normally discharged from a steam source as a dry gas. As steam mixes with cooler duct air, some condensation takes place in the form of water particles. Within a certain distance, the water particles are absorbed by the air stream within the duct. The distance wherein water particles are completely absorbed by the air stream is called absorption distance. Another term that may be used is a non-wetting distance. This is the distance wherein water particles or droplets no longer form on duct equipment (except high efficiency air filters, e.g.). Past the non-wetting distance, visible wisps of steam (water droplets) may still be visible, for example, saturating high efficiency air filters. However, other structures will not become wet past this distance. Absorption distance is typically longer than the non-wetting distance and occurs when visible wisps have all disappeared and the water vapor passes through high efficiency filters without wetting them. Before the water particles are absorbed into the air within the non-wetting distance and ultimately the absorption distance, the water particles collecting on duct equipment may adversely affect the life of such equipment. Thus, a short non-wetting or absorption distance is desirable.
Steam dispersion systems are configured and sized to accommodate for a design condition, also known as the highest load. The appropriate number of dispersion tubes, number of nozzles, and/or orifice size of the nozzles are chosen to achieve the needed non-wetting or absorption distance and the load at the design condition. Some of the current steam dispersion humidification designs use closely spaced tubes with hundreds, even thousands, of nozzles to achieve a short non-wetting or absorption distance at the highest load. Such designs may undesirably heat the duct air and create significant amounts of unwanted condensate. However, if the number of tubes and/or nozzles is decreased, although the heat gain and condensate is reduced significantly, the non-wetting or the absorption distances are increased dramatically at high humidification loads and steam output may be insufficient when the humidification load is at or near 100%.
It is known that when the humidification loads are at or near 100%, i.e., when the humidification demand is at or near 100%, the non-wetting or the absorption distances are increased greatly. The non-wetting or absorption distances become reduced as the humidification load decreases. Although humidifiers are designed to simultaneously accommodate the highest load and an acceptable short non-wetting or absorption distance, humidifiers spend much of their time at loads significantly below 100%. Therefore, the maximum steam dispersion capacity of a system is not normally required to maintain the desired humidity and the non-wetting or absorption distance. Thus, it would be desirable to control or shut off portions of a dispersion system to reduce the air heat gain and condensate when operating at significantly less than 100% load.